1. Field of the Invention:
The present invention is generally related to a control circuit for use with a switched reluctance, or variable reluctance, motor and, more particularly, to a control circuit which has the capability of recovering energy flowing through a stator winding of the switched reluctance motor after a voltage potential has been removed from across the stator winding during the normal operation of the motor.
2. Description of the Prior Art:
In the field of switched reluctance, or variable reluctance, motors, many different types of control systems are known to those skilled in the art. For example, U.S. Pat. No. 4,943,760, which issued to Byrne et. al. on July 24, 1990, discloses a control system for use with a variable reluctance electrical machine. It describes a control system in which the output of a reference wave form generator is applied to a power converter through a current controller. Rotor position, as detected by a sensor, determines the output of the generator. The generator establishes a relative magnitude for rotor phase current for every position of the motor during the period of energization of a motor phase. A further control input may determine the absolute magnitude of the motor phase current, subject to the wave form pattern established by the generator. A method of determining the configuration of a suitable reference wave form for a given motor involves monitoring selected parameters of motor operation while controlling other parameters during a test procedure.
U.S. Pat. No. 4,520,302, which issued to Hill et. al. on May 28, 1985, discloses a stepping motor which has phase windings wherein the current through the phase windings is switched by a drive circuit including switching transistors in series with each individual phase winding. The transistors are controlled by a phase sequence logic circuit and a current flow characteristic is used to indicate the rotor position. The current flow characteristic is used to operate the phase sequence logic circuit. One embodiment of this control circuit incorporates a drive circuit that includes a chopper-type control operated through additional transistors in series with each phase winding. This drive circuit may alternatively provide a constant voltage power supply to the phase windings in sequence with current being repeatedly switched at a low level to an unenergized winding to monitor rotor position.
U.S. Pat. No. 4,611,157, which issued to Miller et. al. on Sept. 9, 1986, discloses a switched reluctance motor drive operating without a shaft position sensor. The switched reluctance motor is operated without requiring a shaft position sensor and the circuit of this patent allows dynamic increase of the torque margin as the dwell angle of the phase circuits is changed in response to a change in load torque. Average current supplied by the DC link is sensed as a measure of load torque and is used to control the dwell angle.
U.S. Pat. No. 4,772,839, which issued to MacMinn et. al. on Sept. 20, 1988, describes a rotor position estimator for a switch reluctance motor. The indirect position estimator described in this patent is for a switched reluctance motor and it applies short duration, low level sensing pulses to two energized phases of the motor. A change in the phase current during a sampling period, resulting from the application of the sensing pulse, is sensed and processed to produce an indication of a pair of estimated angles for each of the unenergized phases. A pair of phase angles for one such unenergized phase is shifted by a value equal to the phase displacement between the two unenergized phases and the shifted angles are then compared to the angles of the second phase to determine which angles match. The matching angle is produced as a signal indicative of the estimated instantaneous rotor position.
U.S. Pat. No. 4,868,478, which issued to Hedlund et. al. on Sept. 19, 1989, discloses a motor energizing circuit for a reluctance motor having at least two phases. A sensing and control circuit is constructed to indicate the inductance or a clearly related magnitude in the winding in the phase which is next to be energized or activated. The circuit is also constructed to detect the current in the winding in the energized phase and to correct the indicated value of the inductance or related magnitude with an operating parameter which has a dependency on the current value in the winding of the energized phase and to compare this corrected value with a reference value and to utilize the rotational position of the motor rotor, when the corrected value coincides with the reference value, as a starting point for determining a rotational position for changing the energizing state of the phase winding.
The patents described above, and many other patents and technical articles known to those skilled in the art, indicate the extent to which different control circuits have been designed to control the operation of variable reluctance, or switched reluctance, motors. Depending on the particular intended use of the variable reluctance motor, a wide variety of control circuits are available. However, because of the way a switched reluctance motor operates, and because of the inductive characteristic of its phase windings, a certain amount of energy is wasted during the sequential energization and deenergization of a stator winding. Depending on the particular control algorithm used to operate the switched reluctance motor, the induced current flowing through an energized stator winding is generally discontinued as a rotor pole moves into alignment with the stator pole associated with the stator winding. Otherwise, the magnetic attraction provided by the energized stator pole would actually inhibit the rotation of the rotor past its alignment position. When a voltage potential is disconnected from the energized stator winding, the inductive characteristic of the stator winding resists a sudden cessation of current flow through it. The current continues to flow and diminishes at a rate determined as a function of the inductance of the stator winding. Various techniques are used to accommodate this inductive characteristic, including the provision of alternative secondary circuits that permit the continuing current flow to recirculate and dissipate. It should be realized that any type of accommodation made for this continued current wastes energy unless the current is used in some way to assist the continued rotation of the rotor. It would therefore be helpful if some method or apparatus is provided to utilize this continuing current that results from the inductive characteristic of the switched reluctance motor windings, especially if the motor is used in conjunction with an apparatus, such as an automobile, where increased efficiency can directly reduce fuel consumption. The present invention provides a control circuit that utilizes this continuing current that flows after the phase winding is disconnected from a voltage potential. The present invention, which will be described in greater detail below, utilizes the continuing current to charge a device which stores the energy for later use in energizing another stator winding of the switched reluctance motor.